Spatial and contrast information are first processed by the visual system within the retina. The processing of these aspects of the visual scene depends on the center-surround, receptive field organization of retinal ganglion cells. While it is known that activation of the concentric regions of the receptive field produces opponent responses in ganglion cells, the retinal circuitry responsible for this receptive field organization remains poorly understood. The center signal is carried by the input-output pathway consisting of photoreceptors, bipolar cells and ganglion cells. However, the lateral pathways that mediate surround signals remain unclear. Conflicting evidence suggests that either horizontal cells in the outer retina or amacrine cells in the inner retina mediate surround signaling. The relative roles of these pathways in ganglion cell receptive field formation are poorly understood. Different features of the visual world are extracted by 10-15 different ganglion cell types, which are then sent to the brain in distinct processing streams. The receptive field organization of these distinct ganglion cells may be uniquely shaped by different inner and outer retinal surround contributions. Ganglion cell receptive fields can also be altered by light adaptation, but it is not known how the inner and outer retina contributes to these changes. The goal of this proposal is to determine the circuitry and synaptic mechanisms responsible for the receptive field variety found across ganglion cells. Electrophysiological, pharmacological and anatomical approaches will be used to determine how inner and outer retinal surround pathways shape the receptive fields in morphologically distinct ganglion cells. Three specific aims will determine 1) the roles of the outer retina in GC receptive field organization 2) determine the roles of the inner retina in GC receptive field organization and 3) determine how light adaptation modifies the inner and outer retinal contributions to GC receptive field organization. By defining the precise roles of inner and outer plexiform layers that mediate surround signaling in specific GC types, we will develop a more complete picture of how spatial and contrast information is processed by distinct visual signaling pathways.